Like every other combat ship, it's niche is just another "flavor of the month" new combat vessel that'll fall to the wayside of obviously superior and more well rounded ships like the FDL, Chieftain, and FAS. All of which share several things: excellent effective HP, great maneuverability, and enough hardpoint damage to smash anything in its way while still flying circles around enemies.
Take one look at the Fed Dropship, Gunship, and the Chieftain's not-quite-right siblings. All of them are nothing more than combat vessels but aren't used because they're simply not as good nor as interesting.
Trying to use them in any other role (BECAUSE Fdev loves designing ships and slating them to a niche by WEAKNESSES and NOT strengths) is mostly a miserable failure. You can't take a Gunship more than a few jumps without sacrificing its already limited module slots. The Dropship is arguably the one that stands out the most here, having borderline multirole capacities of modules but still is hampered by its low maneuverability and relative firepower compared to the other ships. It makes a poor multirole due to its poor fuel and jump range as well.
Similar aspects apply to the Challenger and Crusader. They took what was good from the Chieftain and then butchered it. FDev's design balance when it comes to new designs and iterations of ships is arbitrarily gimping several aspects at once while adding one new thing to it.
In the case of the Challenger, it sacrifices a large hardpoint for two additional mediums; a net gain of 1 hardpoint. It does have one additional internal module slot however, but the problem is that the remaining slots were downsized to compensate, making it useless for fuel, transportation, and quite literally anything else. It's also slower despite having marginally more health, but also has less energy to damage efficiency via the loss of the large hardpoint.
It makes for a gunboat combat vessel that can hold a lot of guns, maybe good with missiles and multicannons but it will still be worse against maneuvering and pinpointing damage against larger vessels like the Chieftain.
The Crusader is the joke of the triplets. With one less medium hardpoint than the Challenger but the ability to launch an SLF, it remains just another flavored combat vessel that is further limited by having less options available to it. It has the same internal modules (roughly) but still can't do more than launch an SLF and use its weaker firepower in conjunction with it.
With all this taken into consideration, the conclusion is that EVERY SHIP HERE does the exact same job, with the only differences really being whether or not they're effective or not. As such, the Federal Vessels remain distinguished in that they have at least some variation between them, but the Alliance derivatives are very poor showings of very poor balance and design. They do not excel; they aren't even side grades. They're poor variations because they have none of the strengths of the original vessel, because those all those strengths were sacrificed for token improvements that ultimate do nothing to aid them.
Bearing all this in mind, the Mamba suffers identical issues as another "derivative" vessel, just with a different mesh. We will be ignoring heat for the moment but it's still one of the BIGGEST issues with the vessel, but it is tangential.
First off it's as large as a Python, but has the internals of an FDL. It holds LESS overall optional module sizes than the FDL despite being much larger. The net addition of two small hardpoints (2 meds = 1 large) over the FDL does not seem reasonable to compromise everything else within it. It has the same utility slots and almost the same weight and shielding scaling. It also, frustratingly has the same FSD. BUT ALSO THE SAME TINY FUEL TANK.
Putting it all together, it's simply ANOTHER FDL in function, combat performance aside. It's simply ANOTHER combat vessel that can't do anything else but coast around a RES for 3 hours before it runs out of fuel.
Ultimately this highlights multiple issues with the balance of combat vessels and the Mamba.
As of now, PVE combat does not have enough variation for the SHEER amount of combat vessels that currently exist in the game. It creates bloated, arbitrary choice because there are very obviously superior choices to take. Even if the alternate choices are taken for novelty, they aren't fun because they suffer from so many issues at once that make their effectiveness much lower on the combat scale. This is on top of uninspired and copied design that fails to consider strengths in ships, favoring weaknesses in order to create choices instead of making them exceptional, with absolutely uncomrpomising balance on making them garbage at anything other than the role they're dedicated at. To which they often fail to be exceptional in their slated roles!
Which makes me ask the question: why aren't these ships dramatically more varied in design? The Federal ships have enough variation to be distinguished into different roles. Different handling, various module slots and varying hardpoint layouts that can alter their usage. But that's just one aspect.
Why aren't some of these combat ships getting LARGER fuel tanks at hte very least? Why is the Mamba stuck to the SAME weight as the FDL, and subsequently the SAME FSD and the SAME fuel tank?
Modern military vehicles have massive fuel tanks to compensate for their high fuel usage as they require high performance all the time. This aspect ALONE is the reason why most of these vessels have so little usage. They simply can't do anything because they can't go anywhere to do it without comrpomising their own vessel for either fuel or combat. It's not choice. It turns ships into little more than toys that you keep at a specific area and ignore otherwise. Giving the Mamba, heck, even the FDL (the latter of which is a slated bounty hunting and transportation vessel, even for the rich) an 8 ton fuel tank with an undersized FSD is like giving a Bugatti Veyron the fuel tank of Prius. The Veyron has a 25 gallon + fuel tank (100 liters.) The Prius has only 11 gallons (roughly 43 liters.) Which begs the question: would giving literally any of the "heavier" combat/multi role'd combat vessels (Dropship, Gunship, Mamba, Challenger, just to name a few) a size higher fuel tank really have any negative impacts in not only the balance of ship vessels but their overall usability, whether being "too good" or otherwise?
The design and limitations of these combat vessels give them no niche except for very generic ones in combat. This is in part due to the lack of variation in combat, and simply the low amount of usable slots that nearly all combat vessels have. You can't take these ships outside of a small range, you can't do anything other than combat with them, and stripping out their few precious module slots for any sort of utility costs them significantly in effectiveness in combat.
In short, what's wrong with these vessels is their lack of versatility that's hardlocked into them (FSD, fuel in particular), combined with lack of actual modularity (module slots) to even MAKE UP for these weaknesses. Military module slots were a great way to improve most of their defensive capabilities, but even their internal slots still remain heavily lacking in function and capability.
The Mamba gets the shortest end of the stick here. With an undersized FSD, a massively undersized fuel tank, and limited module slots for almost anything, on top of its current weaknesses, it has almost no role outside a gimmick of a ship that can't go anywhere and do little. It's a really nice looking ship to fly at high speeds and cruising...for two hours. So no exploration, limited combat capabilities (jousting means taking more damage than avoiding), and a ship that can't go anywhere and is the biggest example of everything wrong with niches in Elite in their current state. Effectiveness in combat aside, it has no role outside as a combat toy.